In Japanese Unexamined Patent Publication No. H.8-258538, the present inventors previously proposed a device for switching air passages in a vehicle air conditioner by means of a sliding door of this kind. And on the basis of this related art, the present inventors produced for trials and studied an air passage switching device shown in FIGS. 10A-11, 13. FIGS. 10A and 10B are sectional views on the line 10—10 in FIG. 3, FIG. 11 is a sectional view on the line 11—11 in FIG. 10A, and FIG. 13 is a sectional view on the line 13—13 in FIG. 3.
In the present trial device, a sliding door 26 has a substantially flat, rectangular door plate 26a and a resin film member 26b supported by this door plate 26a. A flow of air passes through openings 26c provided in the door plate 26a (see FIG. 5) and blows in the arrow b direction onto the film member 26b. A draft pressure of this airflow acts on the film member 26b. 
This draft pressure causes the film member 26b to press against edge seal faces 22b, 23b (FIGS. 10B, 13) of a port 22 or 23 of a case 120, the port 22 or 23 can be closed. And, by the sliding door 26 being slid in the arrow a direction and the film member 26b thereby being moved away from the port 22 or 23 in the case, the port 22 or 23 can be opened. By this means, it is possible to effect air passage switching.
When, as mentioned above, a film-type sliding door 26 was actually produced for trials and studied, it was found that if the opening area of the ports 22, 23 in the case is large, when the port 22 or the port 23 is closed, the film member 26b greatly bends and enters the closed port 22 or 23 under the draft pressure in the passage behind it. When this happens, a gap forms between the edge seal faces 22b, 23b of the ports 22, 23 and the film member 26b. Accordingly, problems arise such as draft leakage and the bent portion of the film member 26b biting into the edge corners of the ports 22, 23 when the sliding door 26 are moved. Accordingly, the force required to operate the sliding door 26 consequently increasing.
As a study into overcoming this, grill members 22a, 23a were formed midway across the ports 22 and 23 on the case 120 side, extending in the sliding direction a of the sliding door 26 and dividing the opening faces into two, to suppress bending of the film member 26b. However, when all that is done is simply to form the grill members 22a and 23a, problems arise such as that the film member 26b impacts the grill members 22a, 23a and produces a striking noise.
Accordingly, the present inventors conceived a solution wherein elastic pressing members 26e are disposed between the door plate 26a and the film member 26b and at all times the film member 26b is pressed by elastic reactions of these elastic pressing members 26e against the edge seal faces 22b, 23b of the ports 22, 23 and the end faces of the grill members 22a, 23a. 
By this means it is possible to prevent, with the grill members 22a, 23a, the phenomenon of the film member 26b bending greatly and entering the ports 22, 23. It is also possible to prevent with the elastic pressing members 26e problems such as striking noises caused by impacting of the film member 26b onto the grill members 22a, 23a. 
However, when the operating characteristics of this device produced for trials were evaluated under actual usage conditions, it became clear anew that disagreeable extraneous noise is produced from the film member 26b. And when the mechanism by which this extraneous noise is produced was investigated in detail by experiment it was found to be as follows.
The case 120 forming the ports 22, 23 is molded from resin, and for reasons relating to the molding process it is molded in two portions. D in FIG. 3 denotes the plane on which the case 120 is divided, and in the example shown in FIG. 3 this dividing plane D is set along the grill members 22a, 23a in a position in front of the grill members 22a, 23a. Thus, the case 120 is made by joining together with fastening means (not shown) left and right half-cases 121, 122 shown in FIG. 3.
Because they share this kind of dividing plane D, the cross-sectional shape (the cross-sectional shape in a W-direction section, orthogonal to the door sliding direction a) of each of the half-cases 121, 122 is U-shaped. And consequently, the portions of the half-cases 121, 122 proximate the dividing plane D fall in toward the inside of the case due to ‘sinking’ of the material after the resin molding.
FIG. 10A shows the assembled state of the sliding door 26 when these portions proximate the dividing plane D have fallen in toward the inside of the case. When this kind of assembled state arises, at the portions in the vicinity of the dividing plane D in the middle of the ports, due to the phenomenon of falling in mentioned above, the spacing L1 between the end faces of the grill members 22a, 23a and the upper face of the door plate 26a become smaller than the spacing L2 at the W-direction ends of the ports 22, 23 (L1<L2).
Consequently, at the portions in the vicinity of the dividing plane D in the middle of the ports, the film member 26b is pressed more strongly than elsewhere. As a result of this, as the sliding door 26 is repeatedly moved back and forth and a concave permanent distortion develops in the film member 26b. 
At the intermediate portions of the film member 26b between the grill members 22a, 23a and the W-direction ends of the ports 22, 23 (the section 11—11 in FIG. 10A), because the pressing force of the elastic pressing members 26e does not act directly on the film member 26b at this position, when the film member 26b passes over the edge seal faces 22b, 23b of the ports 22 and 23, the above-mentioned concave permanently distorted part springs up and warps and makes noise (a popping noise).
FIG. 11 (a sectional view on 11—11 in FIG. 10A) shows the film member 26b passing over the edge seal faces 22b, 23b of a central grid member 123 of the case 120. The above-mentioned popping noise arises both when the film member 26b passes over the edge seal faces 22b, 23b of the central grid member 123 and when it passes over the edge seal faces 22b, 23b at the door sliding direction a ends.
To overcome this, the present inventors produced a device wherein elastic pressing members 26e were added at intermediate positions on the film member 26b, between the grill members 22a, 23a and the W-direction ends of the ports 22 and 23, to increase the pressing force on the film member 26b. With this it was found that by increasing the pressing force it is possible to prevent the above-mentioned warp deformation of the concave permanent distortion and thereby prevent the production of popping noise.
However, instead, an extraneous noise (a crackling noise) caused by an increase in the frictional force on the film member 26b arises. That is, when a predetermined dust-resistance endurance test in an environment wherein dust is mixed with the delivered air (the endurance test conditions being 20,000 back-and-forth movements of the door under the F3 conditions of JIS D0207) is carried out, the surface of the film member 26b roughens, the surface roughness of the film increases from an initial 0.29 μm RZ to 0.61 μm RZ after the test, and this increase in surface roughness combines with the increase in pressing force to raise the frictional force on the film member 26b. 
As a result, the surface of the film member 26b undergoes a so-called slip-stick phenomenon, a sawtooth-form fluctuation in friction caused by microscopic slipping and sticking between the film member 26b and the edge seal faces 22b, 23b on the case side (see part X in FIG. 13), and the film member 26b produces a crackling noise.